The biochemistry and physiology of the renin-angiotensin system will be examined in vitro and in vivo in experimental animals and in man in relation to circulatory homeostasis. The molecular mechanism of renin release will be investigated using a rat renal cortical slice preparation and isolated renin-containing granules. The effects of various peptide hormones, including angiotensin II (AII), bradykinin and substance P, on basal and catecholamine-stimulated renin release will be examined. The interactions of cyclic nucleotides and calcium, putative second messengers in the control of renin release, will be studied during basal, stimulated and inhibited states of renin secretion. Renin from rat kidney cortex, isolated granules and plasma will be partially purified and characterized in order to determine whether it is released in part as an inactive proenzyme under basal conditions. Partially purified rat renin will be used to develop a radioimmunoassay to measure renin directly. The direct renin assay will be used in conjunction with the assay for renin activity to re-evaluate the stimulation and inhibition of renin release from in vitro preparation and to examine the problem of "card activation" of renin. The mechanism of action of converting enzyme will be studied using potent pressor analogs of angiotensin I (AI). The metabolic pathways of lysyl bradykinin, bradykinin, AI and other vasoactive peptides will be examined in detail in the intact dog kidney and in single microperfused nephrons of the rat, particularly to determine whether AI conversion occurs at the brush border of the proximal tubule. Detailed studies of disorders of AI conversion and possible physiologic regulatory mechanisms for converting enzyme in man and in animal models will be performed. The role of the heptapeptide angiotensin III (AIII) in pressure and volume homeostasis will be explored. A sensitive and specific assay for AIII will be developed by combining affinity chromatography and radioimmunoassay and used to measure AIII in plasma and interstitial fluid in various physiologic and pathophysiologic states.